A direct modulation type frequency modulation (FM) transmitter is a transmitter without a mixer. The direct modulation type FM transmitter uses a frequency synthesizer to directly modulate a baseband signal (e.g., a digitally encoded audio signal) to a carrier or a central frequency. In order to modulate the baseband signal to a frequency modulated signal, a fine code associated with the baseband signal may be processed by a digitally controlled oscillator (DCO) of the frequency synthesizer to control a variable capacitor array (e.g., a fine array) of the DCO so that an appropriate modulation frequency of the baseband signal can be generated.
However, an integral nonlinearity (INL) may be introduced to the variable capacitor array due to a mismatch in capacitive elements of the variable capacitor array and/or dependence of the modulation frequency to an inductor-capacitor (LC) circuit of the DCO, where the INL may be a term describing a difference between an ideal frequency and actual frequency of the frequency modulated signal. This nonlinearity may in turn affect total harmonic distortion of the baseband signal.